The present invention generally relates to optical information storage units, and more particularly to an optical information storage unit which is suited for a high density information recording and reproduction.
In this specification, an "information storage unit" refers to an apparatus which records information on and/or reproduces information from a recording medium.
Recently, there is much attention on a recording and reproducing technique which uses the near field when magneto-optically recording and reproducing information on and from a recording medium. Active research is made particularly in a technique which uses a solid immersion lens (hereinafter referred to as a solid immersion lens (SIL) technique), because the SIL technique can realize a super high density information recording and reproduction. The SIL technique itself is disclosed for example in "Current Status & Future of Magneto-Optical Disks" by Nobutake Imamura, MO DISK, Japan, Aug. 21, 1996, pp.54-55.
FIG. 1 is a cross sectional view for explaining the operating principle of the SIL technique. In FIG. 1, a bundle of rays emitted from a light source (not shown) is once converted into parallel rays by a collimator lens (not shown) or, is converted into a convergent spherical wave by a non-aberration lens 105 which can converge the bundle of rays to a diffraction limit in a state of a direct divergent light. The convergent spherical wave is incident to a spherical surface side of a hemispherical lens 106 which is made of a material having a large refractive index, and converges on a surface cut along a equatorial plane of the hemispherical lens 106.
A magneto-optic recording medium 107 is made up of a substrate 107a, a magneto-optic recording layer 107b, and a protection layer 107c. A gap between the hemispherical lens 106 and the magneto-optic recording layer 107b is maintained less than or equal to a wavelength of the bundle of rays irradiated due to an air flow. Because the refractive index of the hemispherical lens 106 is extremely large, a large portion of the bundle of rays undergoes a total reflection if no object exists in a vicinity of the convergent surface. However, if an object exists in the vicinity of the convergent surface, the bundle of rays is radiated due to an evanescent wave coupling.
A spot size of a light beam at an emission surface of the hemispherical lens 106 is inversely proportional to a product of the refractive index and a numerical aperture (NA) of the non-aberration lens 105. In other words, the larger both the refractive index and the NA are, the smaller the spot size of the light beam. For example, if the refractive index is 2, the spot size of the light beam becomes one-half by the provision of the hemispherical lens 106 as compared to a case where only the non-aberration lens 106 having the same NA is provided.
As described above, the SIL technique is an extremely important technique for realizing the super high density information recording and reproduction. For this reason, in order to further improve the recording density, it is desirable to apply the SIL technique to a magneto-optic recording medium having a land and a groove to be recorded with information.
When recording the information on the land, the groove exists between two adjacent lands, and the generation of a crosstalk in which the information recorded on the adjacent land mixes into the reproduced information is suppressed because the lands recorded with the information are separated from each other. Similarly, when recording the information on the groove, the land exists between two adjacent grooves, and the generation of a crosstalk in which the information recorded on the adjacent groove mixes into the reproduced information is suppressed because the grooves recorded with the information are separated from each other.
However, when recording the information on both the land and the groove, information recording regions become contiguous and the generation of the crosstalk from the contiguous recording regions is unavoidable, thereby greatly affecting the information reproducing characteristic.
A method of reducing the crosstalk from the land or the groove by appropriately setting a depth of the groove has been proposed in a Japanese Laid-Open Patent Application No.8-7357, for example. According to this proposed method, in a case where the wavelength of the light beam is 680 nm, the NA of an objective lens used is 0.55 and the widths of the land and the groove are 0.7 .mu.m, the crosstalk is reduced by setting the depth of the groove to approximately 1/6 the wavelength.
But even if the spot size of the light beam irradiated on the recording surface of the magneto-optic recording medium is the same, as the track pitch becomes narrower by the use of the SIL technique described above, there was a problem in that the crosstalk cannot be reduced sufficiently in the case of a groove having the depth on the order of approximately 1/6 the wavelength. In addition, compared to the normal case where the depth of the groove of the magneto-optic recording medium is 1/8 the wavelength, there also were problems in that a carrier level of the signal decreases and a level of a push-pull signal which is used as a tracking error signal also decreases in the case of the groove having the depth on the order of approximately 1/6 the wavelength.
On the other hand, it has been reported that the condition for reducing the crosstalk in the proposed method, that is, the setting of the depth of the groove of the magneto-optic recording medium, easily changes depending on the Kerr ellipticity, spherical aberration and focal error of the objective lens, and the like.
Furthermore, a Japanese Laid-Open Patent Application No.9-128825, for example, proposes a method of reducing the crosstalk by setting the width of the land approximately the same as the width of the groove having an optical depth of approximately 1/8 the wavelength when recording the information on the land and the groove of the magneto-optic recording medium. But in this case, it is necessary to provide two read channels, namely, a read channel for reproducing the signal from the land and a read channel for reproducing the signal from the groove, and it is also necessary to provide a plurality of wave plates and prisms or the like for separating the bundle of rays. Consequently, there was a problem in that the optical information storage apparatus according to this proposed method becomes expensive.
On the other hand, even in the case where the information is recorded on the land or the groove of the magneto-optic recording medium, a reflected light component from the adjacent groove or land increases thereby introducing a phase error as the track pitch is further reduced by employing the SIL technique. As a result, there was a problem in that the information cannot be reproduced satisfactorily from the magneto-optic recording medium because the polarized state changes due to the phase error.